Silicon nitride obtained by sintering silicon powder under a nitrogen atmosphere has the useful property that it withstands high temperatures. It has been shown that the operation of nitriding silicon requires a very low partial pressure of oxygen to avoid stabilization of a surface layer of silica on the particles of the initial powder. This low oxygen requirement is met by adding hydrogen to the nitriding gas or by adding substances, in particular aluminium, to the initial powder, said substances acting as oxygen traps. Further, adding aluminium after preliminary hot oxidation ensures that parts manufactured therefrom are highly resistant to oxidation. This is described in French patent application No. 79 00268 of Jan. 4, 1979 (published under the No. 2,445,747) corresponding to U.S. Ser. No. 109,713.
Varying the oxygen content of the nitriding gas as a function of the quantity of aluminium added to the initial silicon powder controls the development of the microstructure of parts during sintering, and makes it possible to obtain parts containing only one solid phase, (see Journal of Materials Science, 11, 1976, p. 1725-1733). However, it is very difficult to control the oxygen content of the nitriding gas at the required low-pressure levels (about one tenth of a millibar) in industrial installations. Indeed industrial furnaces with metal heating elements are unsuitable since they cannot withstand the action of oxygen entrained by the nitriding gas. As for furnaces with graphite elements, the chemical equilibrium of carbon, oxygen and carbon oxides keeps the oxygen partial pressure therein to very low limits (about 10.sup.-.intg. millibars). Under such conditions, the development of the microstructure of the parts during sintering cannot be controlled by acting on the oxygen content of the nitriding gas.
The present invention aims to provide a method in which it is possible in an industrial furnace to control the development of the microstructure of parts made of silicon to which aluminium is added, and consequently to produce homogeneous nitrided silicon parts of low porosity while retaining the advantages obtained by adding the aluminium to the initial silicon, in particular the resistance of the parts to oxidation.